The present invention relates to a gradient magnetic field generation apparatus adapted for use in an magnetic resonance imaging apparatus and generating a gradient magnetic field for spatially sloping a magnetic field intensity.
In recent years, a very high speed imaging method typical of an EPI (Echo Planar Imaging) method has being put to practical use. Here, the problem with the method is that it is difficult to enable a very strong gradient magnetic field to rise or decay at a very high speed. It is required that, for example, the EPI method, being compared with an ordinary spin echo method, have its magnetic field intensity increased by a factor of 2 to 3 and its rise time shortened by about 1/3 to 1/10.
In order to meet these requirements, an assistant power is used as shown in FIGS. 1 to 4. During a rise period, the main power, together with an assistant power, sharply raises the magnetic field.
In addition to this method, a method is also designed to raise the gradient magnetic field through the utilization of the resonance of a high frequency coil and capacitor. This method can raise the gradient magnetic field with a waveform approximate to a sinusoidal or spiral waveform.
Here, as the gradient magnetic field rising method use is made of a constant slew rate control method for enabling a rise to be achieved at a constant slope as shown in FIG. 5 and a constant rise time control method for achieving a rise at a constant time as shown in FIG. 6. The constant rise time control is characterized in that calculation, etc., becomes simpler at an oblique time but that a circuit arrangement becomes more complicated than the constant slew rate control method. That is, in order to effect constant slew rate control, the output voltage of the assistant power is not varied but, in order to effect constant rise time control, must be varied in accordance with a target coil current.
Thus, the apparatus of FIG. 1 cannot perform constant rise time control. As shown in FIG. 7, therefore, a plurality of assistant powers HV1, HV2 and HV3 are prepared and, through a combination with these assistant powers, a necessary assistant voltage is usually obtained.
In order to vary the assistant voltage to effect the constant rise time control, the problem arises as will be set out below.
1) It is necessary to initially determine the level of the assistant voltage required for the coil current to rise to a largest level with a given rise time. Further, when the level of the assistant voltage is to be determined in real-time, the starting of the assistant power will be delayed behind the main power by a time required for this calculation. PA1 2) The delay time for electric current to actually flow through the gradient coil with respect to the input timing of an input waveform signal will vary, by the restricted frequency characteristic of the main power (linear amplifier), depending upon the assistant voltage. PA1 3) The variation of the delay time in 2) becomes greater depending upon the variation of the response characteristics of a plurality of semiconductor switches for varying the arrangement of the circuit. PA1 4) The switching of one of the semiconductor switches from ON to OFF and that of another semiconductor switch from OFF to ON are done while involving their transient responses of the order of a few microseconds. For this reason, if these switchings are done at a time, then the assistant power becomes dead shorted. PA1 a gradient coil; PA1 means for generating first and second control signals in accordance with the input waveform signal, the first control signal being the same as, or similar to, the input waveform signal; PA1 a delay circuit for delaying the first control signal by a first delay time; PA1 a delay circuit for delaying the second control signal by a second delay time; PA1 a main power for amplifying the delayed first control signal and variably applying a voltage to the gradient coil; PA1 an assistant power for generating an assistant voltage for assisting the main power in accordance with the delayed second control signal; and PA1 delay control means for individually controlling the first delay time and second delay time. PA1 a gradient coil; PA1 a main power for variably applying a voltage to the gradient coil in accordance with the input waveform signal; PA1 a plurality of assistant powers each generating an assistant voltage for assisting the main power, the assistant voltage being varied in a step-like manner in accordance with a corresponding combination with the assistant powers; PA1 a plurality of switches provided for changing the combination of the assistant powers; PA1 means for generating control signals for supply to the switches, there being a variation in the response times of the switches to the control signals; and PA1 at least one delay element provided at least one of the switches so as to compensate for the variation. PA1 a gradient coil; PA1 a main power for variably applying a voltage to the gradient coil in accordance with the input waveform signal; PA1 a plurality of assistant powers each generating an assistant voltage for assisting the main power, the assistant voltage being varied in a step-like manner in accordance with a combination of these assistant powers; PA1 a plurality of switches provided for varying a combination of the assistant powers; and PA1 means for generating a plurality of control signals relative to the switches, the switches being placed in an OFF state when the control signal is at a first level and in an ON state when the control signal is at a second level, wherein PA1 when a first one of the plurality of switches is switched from an OFF to an ON state and a second switch is switched from an ON to OFF state in synchronism with the first switch, the control signal generating means delays a timing for changing the control signal relative to the first switch from a first to a second level behind a timing for changing the control signal relative to the second switch from the second level to the first level. PA1 a gradient coil; PA1 a main power for variably applying a voltage to the gradient coil in accordance with an input waveform signal; and PA1 a plurality of assistant powers each generating an assistant voltage for assisting the main power; and PA1 a plurality of switches, wherein PA1 a circuit including the gradient coil, assistant powers and switches is so configured as to, through the changing of a combination of those ON switches, select an assistance and no assistance of the main power by the assistant powers, combination of those assistant powers assisting the main power and polarities of the assistant voltages.